1. Field of the Invention
The present invention relates in general to identifying sensitive or performance limiting areas or suspect locations of semiconductor devices, and more specifically to a time resolved radiation assisted device alteration technique for determining timing information relative to a test pattern of suspect locations of a device under test using asynchronously scanned radiation stimulation such as a laser.
2. Description of the Related Art
A variety of radiation-based stimulation circuit testing techniques are known for failure or performance analysis of semiconductor devices. Radiation stimulation involves the use of various forms of radiation or emissions or the like with sufficient energy to modify operating behavior of semiconductor circuitry for the purpose of identifying potential problem areas of a semiconductor device. These problem areas are “suspect locations” which includes performance limiting areas or defective circuits and the like. Although many types of radiation may be used, it is desired that the radiation convey sufficient energy to modify circuit operation for purposes of testing the limits of circuit operation. A laser beam, for example, is capable of conveying a significant level of power without damaging semiconductor circuitry and thus is often the radiation of choice for testing. The circuit modification may be any one or more of multiple types, such as modified timing of a device (e.g., transistor, gate, node, etc.), modified voltage level, modified current level, modified resistance, etc. A timing adjustment may reveal, for example, a race condition between two or more circuit paths thereby limiting maximum frequency of operation of the semiconductor device. Similarly, a marginal voltage or current or resistance level affecting pass-fail behavior may be revealed using radiation perturbation during testing. It is appreciated that many other measurable variations of a device may be monitored for determining pass-fail behavior, such as an input/output (I/O) voltage level or timing, VDD or pin current, output frequency, a signal slew rate, etc.
Laser assisted device alteration (LADA) is a known laser scan technique used in the failure analysis of semiconductor devices. A laser generated by a laser scanning microscope (LSM) or the like is used to alter the operating characteristics of transistors, metal interconnects or other components on the semiconductor device under test (DUT) while it is electrically stimulated. Certain operating characteristics of the laser (e.g., wavelength, laser spot size, power, etc.) may be selected or otherwise adjusted to modify circuit characteristics, such as circuit timing. For example, a laser operating at a wavelength of approximately 1,064 nanometers (nm) produces localized photocurrents within active transistor layers in which the photo-generated currents modify circuit timing. Alternatively, a laser operating at a wavelength of approximately 1,340 nm produces localized heating which also alters circuit timing (e.g., slowing down of logic transitions). It has been observed that photocurrent injection enables significantly larger timing shifts as compared to thermally induced alteration, so that photocurrent injection is more commonly used for “standard” LADA testing. Electrical stimulation of the DUT is usually performed by automated test equipment (ATE) which applies an ATE test loop or test pattern to the DUT and monitors the results. The term “ATE” as used herein refers to any test equipment or electronic device or system or the like which provides electrical stimulation to a DUT and which monitors results. The test pattern is designed by test engineers with multiple test vectors applied in sequential order to perform critical timing testing.
According to an exemplary LADA test technique, the entire area or a selected portion of interest of the DUT is first scanned by the laser at an appropriate imaging power and the reflected image is monitored and mapped to provide a visual image of the topology of the physical circuit. The power of the laser is adjusted to an appropriate level for analysis and applied to the same mapped area of DUT while the test procedure is conducted by the ATE. In particular, the test pattern is applied at a selected clock rate and the results are monitored while the laser is applied to one pixel location of the mapped area. The laser is moved (e.g., slow scan or discrete positioning) relative to the DUT to the next pixel and the entire test pattern is repeated. It is appreciated that the laser beam or the DUT may be moved for laser scanning depending upon the laser equipment as understood by those skilled in the art. One or more test variables, such as laser power, supply voltages, temperature, clock frequency, etc., may also be selected to adjust operation relative to a pass-fail boundary of the voltage-frequency relationship (which may be plotted on a “shmoo” graph to illustrate the pass-fail boundary as known to those skilled in the art). In a pass test mode, the variables are selected to adjust timing towards a pass-fail threshold while still in the pass area and the test pattern is applied to determine any fail locations. In a fail test mode, the variables are selected to adjust timing just beyond the pass-fail threshold into the fail area and the test pattern is applied to determine any pass locations. In a pass-fail test mode, the variables are selected to adjust timing on the pass-fail boundary and the test pattern is applied multiple times for each pixel location. The statistics are measured to identify any suspect locations that deviate from the expected test results by a statistically significant or measurable amount. It is noted that “significant” deviations may include subtle or minor deviations determined by averaging a large number values or the like. In a 50/50 pass-fail test in which results of multiple tests are averaged, any locations that deviate from the expected 50% pass-fail rate are identified for additional testing. For example, locations which pass or fail more or less than 50% are considered suspect locations. The LADA analysis has been used in this manner to identify circuit nodes or elements that modulate the pass-fail result.
The next step in the device analysis is signal acquisition, such as, for example, time resolved light emission microscopy (TRLEM) probing. Signal acquisition techniques other than TRLEM probing are known and contemplated. During TRLEM probing, suspect nodes are probed during the test pattern in an attempt to identify the deviation from expected results. The TRLEM technique is used to acquire functional waveforms from the substrate in a contactless manner through the back of the die. For example, infrared light is used since silicon is transparent to this wavelength so that infrared light escapes through the back of the die. The light versus time waveform is a function of the device current, device voltage, device temperature, etc. through the transistor or component being probed. Thus, the current through a device is indirectly measured to identify when it switches and the voltage and logic levels versus time can be determined from other more subtle characteristics of the light versus time signal. TRLEM is analogous to an oscilloscope and other probing techniques.
The conventional LADA test procedure is predominately empirical in nature since although the suspect location is determined, the timing information is not provided. More specifically, the particular test vector which caused the test deviation during laser excitation at each suspect location is not known. The pass-fail indication is not provided on a vector by vector basis but instead is only provided at the end of each test pattern. The test pattern generally includes hundreds or thousands (or more) of test vectors, so that selection of timing is a time-consuming trial and error approach. The TRLEM procedure without timing information often consumes a significant amount of analysis time with a relatively low success rate (e.g., success rate is estimated to be less than 1 percent). Probe selection is particularly difficult since only one probe point is monitored at a time over a very small window of time relative to the overall duration of the test pattern. As an example, a typical test pattern may have a duration of approximately 20,000 clock cycles (at 1 test vector per clock cycle) and the probe window is only about 20 clock cycles, such that the test pattern duration is one thousand times the size of the probe window.
If nothing unusual or helpful is discovered during the initial TRLEM probing, then backwards TRLEM probing is conducted starting from the external input/output (I/O) pad of the die that indicated the failure. In some cases, this analysis path is made even more difficult as in the case where a built-in self test (BIST) pattern fails and there is no way to accurately predict where in the pattern the statistical deviation event occurred. The loop length can be more than 1,000 times longer than the functional probing time window. This portion of testing, if necessary, consumes a substantial amount of valuable analysis time (e.g., several days or weeks).
At least one method is known for determining spatial and temporal selective laser assisted fault localization. This method requires a pulse laser system with a fully controllable dynamic laser stimulation apparatus connected to a control unit that provides complete synchronization with a tester unit. Synchronized pulse laser systems, however, are not readily available, can be very expensive, and are challenging to integrate into an existing LSM system. The synchronized pulse laser technique requires synchronization between the test pattern and laser pulses which is very complex and difficult to achieve. Thus, the synchronized pulse laser technique has not been widely used.